This disclosure relates to gas flotation equipment and methods used to separate oil from an oily water stream.
Gas flotation equipment and methods use small gas bubbles to “float” oil droplets out of an oily water stream. The gas bubbles can be originally dissolved in one or both phases of the stream (water, oil) and can be added to the oily water stream. Sometimes, the oily water stream passes through two or more stages of this gas flotation treatment. The stages can be housed in the same vessel or in separate vessels. Each stage typically includes an oil outlet, water outlet, and gas outlet, with almost all of the dissolved gas in the oily water stream being released in the first stage.
Within each stage of treatment, the separation efficiency of the oil droplets and the gas bubbles is determined, in part, by the gas bubble diameter relative to the oil droplet diameter. Generally speaking, the smaller the gas bubble diameter, the better the separation of the oil droplets from the oily water stream.
A problem with multi-stage gas flotation treatment is almost the entire the pressure drop occurs between the upstream unit that provides the oily water stream and the first stage flotation separator, leaving little room available for large operation pressure differentials to occur between this flotation separator and later stage flotation separator(s). This first operation pressure differential causes almost all of the dissolved gases are released from the water and oil phases of the stream in the first stage flotation separator.
As the oily water stream passes to the second stage flotation separator, the (not all that much) lower operation pressure of this second separator is relatively ineffective in releasing any remaining dissolved gases trapped in the water or in the remaining oil droplets. The end result is declining separation performance from one stage of flotation treatment to the next, with the first stage of flotation treatment having the best performance.